Conventionally, endurance testing of a pneumatic tire is carried out, according to the standard JIS D4230, by inflating the tire with air to a regulated air pressure, leaving the tire at a temperature of 38.+-.3.degree. C. for three hours while maintaining a constant air pressure, and re-adjusting the air pressure as regulated and carrying out the endurance test. Further, JIS D4230-1978 discloses a testing method for humidity aged durability to investigate aging of a pneumatic steel radial tire for passenger under high-temperature and high-humidity, as a reference for measuring the steel cord separation problem, which typically arises during the summer. This method is carried out by leaving a tire without a rim under the condition of a temperature of 38.degree. C. and a humidity of 95% for 20 days, and then carrying out a drum endurance test according to the high-speed endurance property test in JIS 4230.
The expected insured lifetime of a pneumatic tire is increasing recently as a result of increasing of reliability of the entire vehicle system, so it is inevitable to evaluate endurance after deterioration by aging. In the testing method for humidity aged durability described in JIS 4230-1978, however, the experimental condition such as leaving condition under high-temperature and high-humidity is not definitely set, and it has been found that some cases of failure cannot be reproduced in conventional in-house or out-house endurance tests when the tires suffer deterioration by actual use for a long time.
In this connection, the test method for a pneumatic radial tire has been expected to reproduce deterioration by aging after actual use, that is, to evaluate the endurance property after deterioration by aging appropriately and effectively.